Leonard Alfred Schneider was born in Mineola, New York, grew up in nearby Bellmore and attended Wellington C. Mepham High School. His youth was chaotic; his parents divorced when he was five years old and Lenny moved in with various relatives over the next decade.

His mother, Sally Marr (née Sadie Kitchenberg), was a stage performer who had an enormous influence on Bruce's career. After spending time working on a farm with a family that provided the stable surroundings he needed, he joined the United States Navy at the age of 17 in 1942, and saw active duty in Europe until his discharge in 1946.

In 1947, soon after changing his last name to Bruce, he earned $12 (equivalent to $116 in 2008) and a free spaghetti dinner for his first stand-up performance in Brooklyn, New York. From that modest start, he got his first break as a guest (and introduced by his mother, who called herself "Sally Bruce") on the Arthur Godfrey's Talent Scouts Show, doing a "Bavarian mimic" of American movie stars (e.g., Humphrey Bogart).

In 1951, he was arrested in Miami, Florida, for impersonating a priest. He was soliciting donations for a leper colony in British Guiana after he legally chartered the "Brother Mathias Foundation" (a name of his own invention, but possibly taken from the actual Brother Matthias who had befriended Babe Ruth at the orphanage that Ruth had been confined to as a child), and, unknown to the police, stole several priests' clergy shirts and a clerical collar while posing as a laundry man.

He was found not guilty due to the legality of the New York state-chartered foundation, the actual existence of the Guiana leper colony, and the inability of the local clergy to expose him as an impostor. Later in his semifictional autobiography How to Talk Dirty and Influence People, he revealed that he had made approximately $8,000 in three weeks, sending $2,500 to the leper colony and keeping the rest.

Bruce's early comedy career included writing the screenplays for Dance Hall Racket in 1953, which featured Lenny, his wife, Honey Harlow, and mother, Sally Marr, in roles; Dream Follies in 1954, a low-budget burlesque romp; and a children's film, The Rocket Man, in 1954. He also released four albums of original material on Berkeley-based Fantasy Records, with rants, comic routines, and satirical interviews on the themes that made him famous: jazz, moral philosophy, politics, patriotism, religion, law, race, abortion, drugs, the Ku Klux Klan, and Jewishness.

These albums were later compiled and re-released as The Lenny Bruce Originals. Two later records were produced and sold by Bruce himself, including a 10-inch album of the 1961 San Francisco performances that started his legal troubles. Starting in the late 1960s, other unissued Bruce material was released by Alan Douglas, Frank Zappa and Phil Spector, as well as Fantasy. Bruce developed the complexity and tone of his material in Enrico Banducci's North Beach nightclub the 'hungry i,' where Mort Sahl had earlier made a name for himself.

His growing fame led to appearances on the nationally televised Steve Allen Show, where on his debut Lenny commented on the recent marriage of Elizabeth Taylor to Eddie Fisher by making his first line an unscripted 'will Elizabeth Taylor become bar mitzvahed?'.

He also began getting mainstream press, both favorable and derogatory. Hy Gardner, the syndicated Broadway columnist called Bruce a 'fad' and 'a one-time-around freak attraction,' and Variety declared him 'undisciplined and unfunny.

On October 4, 1961 Bruce was arrested for obscenity at the Jazz Workshop in San Francisco; he had used the word cocksucker and riffed that "'to' is a preposition, 'come' is a verb" and that the sexual context of "come" is so common that it bears no weight, and that if someone hearing it becomes upset, they "probably can't come."

Although the jury acquitted him, other law enforcement agencies began monitoring his appearances, resulting in frequent arrests under charges of obscenity. The increased scrutiny also led to an arrest in Philadelphia for drug possession the same year, and again in Los Angeles, California, two years later.

By the end of 1963, he had become a target of the Manhattan district attorney, Frank Hogan, who was working closely with Francis Cardinal Spellman, the Archbishop of New York. The association of Hogan and Spellman led to the often repeated speculation that Bruce's persecution was actually fueled by his status as the original comedic Catholic Church-basher.

In April 1964, he appeared twice at the Cafe Au Go Go in Greenwich Village, with undercover police detectives in the audience. On both occasions, he was arrested after leaving the stage, the complaints again resting on his use of various obscenities.

A three-judge panel presided over his widely-publicized six-month trial, with Bruce and club owner Howard Solomon being found guilty of obscenity on November 4, 1964. The conviction was announced despite positive testimony and petitions of support from Woody Allen, Bob Dylan, Jules Feiffer, Allen Ginsberg, Norman Mailer, William Styron, and James Baldwin, among other artists, writers and educators, as well as Manhattan journalist and television personality Dorothy Kilgallen and sociologist Herbert Gans.

Bruce was sentenced on December 21, 1964, to four months in the workhouse; he was set free on bail during the appeals process and died before the appeal was decided. Solomon's conviction was eventually overturned by New York's highest court, the New York Court of Appeals, in 1970 (People v. Solomon, 26 N.Y.2d. 621).

On August 3, 1966, Bruce was found dead in the bathroom of his Hollywood Hills home at 8825 Kings Road. The "official" photo, taken at the scene, showed Bruce lying naked on the floor, a syringe and burned bottle cap nearby, along with various other narcotics paraphernalia. His official cause of death was acute morphine poisoning caused by an accidental overdose.

He was interred in Eden Memorial Park Cemetery in Mission Hills, California, but an unconventional memorial on August 21 was controversial enough to keep his name in the spotlight. The service saw over 500 people pay their respects, led by legendary record producer Phil Spector.

Cemetery officials had tried to block the ceremony after advertisements for the event encouraged attendees to bring box lunches and noisemakers. Dick Schaap famously eulogized Bruce in Playboy, with the memorable last line: "One last four-letter word for Lenny: Dead. At forty. That's obscene."